842 CORRESPONDENCE
Inorganic Chemistry
The chemical shifts of the resonances are quite in the transition state during interconversion of consensitive to solvent p r 0 ~ e r t i e s . l ~Hence one notes formers.23-25 that the positions of the resonances a t high tempera(23) The methallyl-molybdenum complex was a yellow solid which melted to a n oil above room temperature and was prepared by a method tures are not exactly those of the weighted averages of similar to that for the allyl complex.24 the low-temperature resonances. This limits the ac(24) hl. Green and hl. Cousins, J. Chefn. Soc., Sect. A , 88Y (1963). (25) We are indebted to the Climax hlolybdenum Corp. for a gift of curacy with which one can calculate exchange rates molybdenum carbonyl, the Petroleum Research Fund for financial support, and the National Science Foundation for the departmental grant which from the line shapes; however, the changes are not supplied the HA-100. so severe that one cannot obtain approximate activaDEPARTMEST OP CHEMISTRY tion energies using the Kubo-Sack matrix method for J. W. FALLER YALEUNIVERSITY M. J . INCOXVIA line-shape analysis.l5 For coniplex I the activation SEW HAVEU,COKNECTICUT 06520 parameters for the rate of conversion of one conformaRECEIVED NOVEMBER 27. 1967 tion to the other were determined by matching the experimental and computed line shapes of the cyclopentadienyl resonances over a temperature range of -5.5 to +9.5" for seven points and gave a value of E , = 12.3 1.2 kcal/mole and log A = 10.6 1.0. The New Conformers of methyl resonance line shape for the methallyl complex (11) was used to determine both the relative populaTris(ethylenediamine)chromium(III) tions and the rates. A least-squares fit of five p o i n t ~ ~ ~ - l ~ between 13.5 and 54.5" gave E , = 16.8 0.4 kcal/ Sir : mole and log A = 14.5 ==I 0.3. These activation paI n the conformational analysis of metal chelates, rameters for configurational interchange without synthe relatively simple tris(ethy1enediamine) complexes anti exchange may be compared with activation enhave received considerable attenti0n.l In these comergies of 9 to approximately 20 kcal/mole for synplexes there are two ways the carbon-carbon bond of anti exchange through a u-bonded intermediatez0 for the ethylenediamine (en) ring can bend to achieve the compounds of the form L2C1zRh(a-C4H7).21*22 U'e unstrained gauche conformation. In the first of these therefore conclude that opposing steric factors rethe carbon-carbon bond is nearly parallel to the threequire that A predominate in the a-allyl complex and fold axis of the metal complex, and in the second it conformer B predominate in the a-methallyl complex. forms an obtuse angle with this axis. For a A conThe apparently greater activation energy for the afiguration2 about the metal, these ring conformers are methallyl complex indicates further steric interactions designated d and X, respectively. The A absolute configuration is defined by assigning it to that isomer of the complex which, when viewed down the threefold axis, (14) The chemical shift of the anti-proton resonance for I1 is highly forms a left-handed propeller. The mirror-image isosolvent dependent: in CSv the major anti resonance is 0.10 ppm downfield from the major methyl resonance, and in CICL i t is 0.03 ppm downfield a t low mer is designated A. This allows the eight possible temperature and merges into the methyl resonance as the temperature is isomers: bddd, ALMA, AdXX, MAX, AXXX, AXXd, AXM, raised. A d N . The last four are mirror images of the first (15) R. Kubo, h'uovo Cimento Suppl., 6, 1063 (1957); R. A. Sack, Mol. Phys., 1, 163 (1958). We are indebted t o Professor Martin Saunders for four, in sequence, and the members of each stereoisosupplying a computer program which carried out the matrix manipulameric pair, of course, have identical conformational tions. (16) Use of the anti-proton resonances was less reliable owing t o impurity stabilities. For this reason only the first set of four interference," but it gave E , = 14.5 i 1.8 kcal/mole and log A 12.5 f 1.3 conformers will be discussed in the following compariand a population ratio of 14.5:l (compared t o 1 6 : l from the methyl resonance). T h e solvent dependence of the chemical shifts in these compounds son of stabilities. makes all of the activation energy calculations somewhat less reliable than Corey and Bailar3 calculated the relative energies normal; nevertheless, i t is felt that the difference between the T-allyl and the a-methallyl compounds is significant. We have assumed t h a t the equiof the iiddd and i i X X X conformations of [Co(en),13+ librium constant for the interconversion of conformers is independent of tem(designated by them as kkk and k'k'k', respectively). perature. This should be valid on the basis of the low-temperature behavior of both the palladium and molybdenum complexes and has also been noted These calculations were based on a comparison of the in nickel-allyl complexes.18 Considering the uncertainties and magnitudes nonbonded interactions in the ring systems which one expects in these Ea calculations, the values of AH (assuming A S 0) of 0.6 and 1.6-1.8 kcal/mole, respectively, for the allyl- and methallyland predicted the d d 8 form to be 1.8 kcal/mole molybdenum complexes suggest t h a t the energy profile is effectively symlower in energy than the XXX form. X-Ray diffracmetric, such that E, was obtained from the observed rates. (17) Difficulty in obtaining a-allylic complexes free from impurities has tion studies of several compounds (Table I) have been noted elsewhere.10 supported this assignment since only the ,383 (18) H. Boennemann, B. Bogdanovic, and G. Wilke, Angew. Chem., 79,817 (1967). conformation was found. The compounds studied in(19) C. A. Reilly and H. Thyret, J . Amer. Chem. Soc., 89, 5144 cluded only Co(II1) and Ni(I1) salts. However, ter (1967). (20) K. Vrieze and H. C. Volger, 3 . Ovganometal. Chem. (Amsterdam), 9, Berg4has shown that the salts [hT(en)3]Cla.3Hz0 (M = 537 (1967).
*
*
*
-
-
(21) We have noted in experiments similar t o those of Becconsall a n d O'Brien2Z that in bis(rr-methal1yl)palladium the temperature dependence is due t o both syn-anti and conformational interchanges and that the line shapes could not be matched by considering the effect of a 0-bonded intermediate mechanism alone. The decomposition occurring in t h e palladium system makes a n y definitive conclusions difficult. (22) J. K. Becconsall and S. O'Brien, J . Oi.gnmmeln1. Cheni. (Amsterdam), 9, 27 (1967).
(1) See. for example, R . D. Gillard and H. M. Irving, C h e m Rev. 66, 603 (1965); J. H. Dunlop and R. D. Gillard, A d v a n . Inoug. Chem. Radiochem., 9, 185 (1966); A. M. Sargeson, "Transition Metal Chemistry," Vol. 3, Mal-cel Dekker, Inc., N e w York, N. Y.,1966, p 303. ( 2 ) The recent IUPAC proposal for nomenclature i s used here. ( 3 ) E . J Corey and J. C. Bailar, J . A m . Chem. .Sot., 81,2620 (1959). (4) J. ter Berg, . W z i k / ? d w i c h t . 7, 2 3 5 (193Y).
CORRESPONDENCE 843
Vol. 7, No. 4 , A p r i l 196‘8 TABLE I CONFORMATIONS OF M(en)3 COMPLEXES Ring conformation”
Compound
P
a
38 23.2 46.2”c 33 1 9 . 7 41.2d 38.18 A - [ C O ( ~ ~ ) ~ ~ C I ~ . ~ . . ~3 N3 ~ C18.9 I.~H~ O 42.W A- [Co(en)a]Br3.0.5HzO 1d 20.7 [Ni(en)31[NO311 [Co(en)s]Cl3.3H20
[Cr(en)~] [Ni(CN)b] 1.5HzO Cation 1 a
/;; $I
1; ;:: 4:; 1d
Cation 2 [Cr (en)a][CO(CN)G] .6HzO
@ ;
26.4
;
;
;
50.8
25.1 49.1” 27.0 51.8 a For the racemic salts this corresponds t o A configuration around the metal. The number refers to the number of symmetry related rings with identical geometry. * Error assignRefments are described in the text. c References 12 and 13. erence 5. 6 References 9 and 10. / Reference 11. Reference 7. h Reference 8. 2X 1X
@
Co, Cr, or Rh) and [M(en)3]Bra.3H20(M = Co or Cr) are all isomorphous. A change from the dad to the XXX conformation involves a general broadening of the metal complex and would be expected to affect markedly the lattice parameters. I n addition, the orientations of the amine hydrogens are strongly affected by a change in conformation. For these reasons the isomorphisni of [ C ~ ( e n ) ~ ] C 3H20 l ~ (space group P%l, a = 11.50 and c = 15.52 A conformation add5) with the Cr(II1) salt ( a = 11.49 and c = 15.52 A)* implies that the ddd conformation is also present in the Cr(II1) complex. In fact, i t has been widely accepted that this conformation is the only one to be expected since in the several compounds previously studied all ethylenediamine rings have had this conformation. We have completed an X-ray study of the structure of [Cr(en),][Ni(CN)s] 1.5H20.6 This unusual salt contains two diflerent five-coordinated [Ni(CN)51aions. One is a perfect square pyramid and the other is a distorted trigonal bipyramid.’ We have also recently completed an X-ray study of [Cr(en)31[Co(CN)6] 6 H ~ 0 .Of ~ the two crystallographically independent [Cr(en)3I3+ ions in [Cr(en)3][Ni(CN)5] 1.5H,O and a third [Cr(en)3I3+ion in [Cr(e11)~1 [Co(CN 6 ] 6Hz0, none of the three has the expected dad conformation; instead each i o n i s a new, diferent conformer so that all three of the previously unobserved conformers (dXA, ddX, and XXX) are represented. The first [Cr-
-
(5) K. Nakatsu. Y. Saito, and H. Kuroya. Bull. Chem. SOC.J a p a n , 29, 428 (1956) These authors report the Co(II1) complex is of continuously variable composition with a maximum of 3.5 waters of hydration. T h e Cr(111)salt is normally written with the latter hydration number. ( 6 ) K. N. Raymond a n d F. Basolo, Inoug. Chem., 6, 949 (1966) (7) K . N. Raymond, P. W. R. CorCield, a n d J. A. Ibers, submitted for publication. (8) K. N. Raymond and J. A. Ibers, submitted for publication. (9) K.Nakatsu, Y.Saito, and H. Kuroya, Bull. Chem. SOC.J a p a n , SO, 158 11957). (10) K. Xakatsu, Y . Saito, and H. Kuroya, ibid., SO, 795 (1957). (11) K . Nakatsu, ibid., SS, 832 (1962). (12) L. N. Swink and M. Atoji, Acta C r y s l . , 13,639 (1960). (13) T. Watanabe and M. Atoji. Kagaku (Tokyo), 21, 301 (1951).
Figure 1.-Perspective drawings of the dXX (a), ddX (b), and XXX (c) conformers of [Cr(en)3lZ+as viewed down the usual molecular threefold axis.
(en)3]3+cation in the [Ni(CN)6]3-salt is shown as a projection down the usual molecular threefold axis in Figure l(a). The conformation is AdXX (that is, only one of the carbon-carbon bonds is nearly parallel to the threefold axis). The second cation in the [Ni(CN)5]3- salt is shown as the same projection in Figure l(h). Here the conformation is AddX. The most unusual conformer occurs in the [Co(CN),I3salt (Figure 1(c)), where the [Cr(en),l3+cation has the supposedly highest energy AAAX form (that is, none of the carbon-carbon bonds is parallel to the threefold axis). Two questions must now be answered. First, why have not these conformations been observed in the previous structures, and, second, what systematic differences, if any, in the crystal environment can be invoked to explain the stabilization of a particular conformer? We believe the answers to both questions lie in the potentially large number of hydrogen bonds in which the amine hydrogens can participate. In ethylenediamine rings with both d and X conformations there are two different kinds of amine hydrogens. One type points more or less in the axial direction and the other points out in an equatorial direction. In both the d and the X conformations, the equatorial hydrogens are free to form hydrogen bonds since they point out like the spokes of a pinwheel. The change from d t o X conformation, however, makes a large difference between the respective axial hydrogens. As may be seen from molecular models, in the dad conformation all of the axial hydrogens are directed toward a point on the threefold axis above the molecule. Since the N-H--B hydrogen bond angles usually are between 120 and 180’ (where B is the base
844 CORRESPONDENCE
Inorganic Chemislry
of the hydrogen bond), all three axial hydrogens of ethylenediamine ring is expected to have a guuche conthe add conformation cannot simultaneously form a hyfiguration and e should differ from zero. I n Table I drogen bond. When a basic atom is in position to we tabulate the values of a for those ethylenediamine bond with one hydrogen, i t effectively blocks the other complexes studied to date. I n all cases the rings have two. In contrast, in the XXX conformation the axial the gauche configuration. We also tabulate in Table hydrogen atoms point away from the threefold axis in I the angle /3 between the two nitrogen atoms as one an outward spiral and so all three amine hydrogens are looks down the carbon-carbon bond. Values of p have free to form hydrogen bonds, Therefore, hydrogen generally been given by previous authors. Our tabulation of p is based a recalculation from the atomic bonding specificully Savors the X conformation relative to parameters given in the various papers. In some cases the normally more stuble a f o r m . In a particular trisdifferences as great as 10" occur between the value we (ethylenediamine) complex salt the aad conformer is then expected to be found when there are few if any calculate and the value given previously. The assigned standard deviations are, in the Cr(II1) case, basic sites in the salt to bind with the amine hydrogens. However, if there is an abundance of such sites, the around 1.1O as estimated from the variance-covariance energy released by the formation of a few extra hydromatrix. The errors for the other complex cations are estimated to be 3-5 times greater on the assumption gen bonds (a fern kilocalories per mole per bond) can that the ratio of bond length errors to angle errors is make the XXX conformer the most stable. Intermediate constant for these very similar molecular geometries. situations might be expected to give intermediate conIn conclusion, the X conformation (for a A configuraformers when the opposing effects (interatomic hydrotion of the [I\I:(en)3]n+complex) is favored in those gen-hydrogen repulsion and hydrogen bonding) cancel. cases where strong hydrogen bonds can be formed. In [Co(en)31Cl3.3H2O5 there are N-H-C1 hydrogen The compounds studied to date indicate this is true bonds but more amine hydrogens than C1- ions, and in the solid state and i t is presumably also true in the shortest PI;-Cl distances are in equatorial directions. This is true again in the salts h-[Co(en)3CI~- solution. For this reason it is probably impossible to predict what conformers will be found in strongly hy0.5NaCl-3H209J0 and A- [Co(en)j]Bra* 0.5Hz0.I1 In drogen-bonding solvents, such as water. The adthe latter case there are both short N-Br and N-0 dition of strongly hydrogen-bonding anions to soludistances and these are all equatorial. On the basis tions may also stabilize specific conformers. This of the hydrogen-bond model proposed earlier, the dda could provide an alternative explanation for the conformation would be expected t o be most stable for observed in circular dichroism spectra upon these salts, as is observed. I n [Ni(en)a][ N 0 3 ] ~ , ' ~ * changes ~~ the addition of such anions.14 the situation is complicated by the formation of bifurcated hydrogen bonds so that each amine hydrogen is Acknowledgment.--This research was supported by associated with a short H-0 distance. However, this the Kational Science Foundation and the National kind of interaction normally is weaker than the usual Institutes of Health. We thank Professor F. Basolo hydrogen bond, as %ink and Atoji pointed out in the for his interest in this work. IVe also thank Professor interpretation of their infrared data. The extra C. F. Liu and Dr. N. C. Payne for calling our attention hydrogen bonds may then be too weak to affect the to the fact that only the 13% conformation had preConformation? or the specific geometric requirements viously becn reported. imposed by the planar NOa- anion may be important. (14) H. L. Smith and R.E. Douglas. Iiioia. Chm., 5 , 784 (1BGG). The hydrogen-bonding model appears to work well (15) National Institutes of Health Predoctoral Fellow 1965-1867. in explaining the stabilization of the higher energy DEPARTXENT O F CHEhlISTRP KENNETH S .R A Y M O N D ~ ~ conformers of [Cr(en)3]3+in the [Ni(CXj6]3- and KORTHVESTERN UXIVERSITP P. W. R. CORFIELD [co(cN)6]3- salts. The complex cyanide anions and EVAXSTOS, ILLINOIS60201 JAMES -4. I B E R S the waters of crystallization provide many possible RECEIVED DECEMBER 13, 1967 sites for hydrogen bonding with the amine hydrogens, so the possibility of stabilizing higher energy conformers certainly exists. Of the three conformers observed, the On the Applicability of the Van Vleck order of increasing energy is expected to be ddX < a A X < XXX. The ddX conformer (Figure l(b)) has only Formula for Polynuclear Complexes three strong hydrogen bonds (N-N or N-0 less than 3.10 A). The ahX conformer (Figure l(a)) has seven, Sir: and the highest energy XXX conformer (Figure l(c)) A recent communication by Wentworth and Sailhas ten bonds in this range, consistent with the exlant' presented magnetic suceptibility measurements pected relative stabilities. on the tetrameric complex [Cr4(en)~(OH)6]Ig.4HzO. Let us define the dihedral angle CY between the plane The interpretation of these results was said to favor which contains the ring carbon atoms and the metal a trigonal structure with a central Cr(II1) atom interatom and the plane which contains the ring nitrogen acting magnetically with three Cr(II1) atoms a t the atoms and the metal atom. This angle would be zero corners of a triangle. The basis of the interpretation if the ethylenediamine ring had the eclipsed configuration. As we pointed out earlier, the coordinated (1) 12. A. 11. Wctltwoith and R. Saillant, I?zorg. Chem., 6 , 1236 (1967). ~
